Teva Pharmaceuticals had petitioned the FDA to allow stores to put Plan B on shelves with other family-planning products -- and to sell it to anyone who wanted it. The company conducted a study it said showed that girls as young as 12 could understand how to safely use the single-pill product.

The FDA agreed. In a statement released today, FDA Commissioner Margaret Hamburg, MD, said "there is adequate and reasonable, well-supported, and science-based evidence that Plan B One-Step is safe and effective and should be approved for nonprescription use for all females of childbearing potential."

Sebelius noted that about 10% of girls are able to get pregnant at age 11. In a statement, Sebelius said her action "reflects my conclusion that the data provided as part of the actual use study and the label comprehension study are not sufficient to support making Plan B One-Step available to all girls 16 and younger without talking to a health-care professional."

The studies submitted to FDA, as published, included girls aged 12 to 17. At least 79% of the youngest girls could understand the instructions.

By the time a girl gets a prescription for Plan B, it may be too late. The drug must be taken within three days -- 72 hours -- of unprotected sex.

"The sooner Plan B is taken, the better it works," Amy Niemann, vice president of Teva Women's Health, tells WebMD. "That is the entire rationale for having widespread availability for this product."

Nancy L. Stanwood, MD, MPH, section chief of family planning at Yale School of Medicine, says full OTC status would have made emergency contraception available to many more people who need it.

"The irony of Plan B not being OTC for women of all ages is that it has not been available for the women who need it the most," Stanwood tells WebMD. "Teens may be sexually active for a while before they see a doctor to get a prescription for contraceptives. ... [With emergency contraceptives] they don't just have to hope the condom doesn't break. There is something they can do."

And there's another group of women that may want emergency contraception: rape victims.

"Many women who have been raped do not come in for medical care, and many do not see a doctor in a timely manner," Stanwood says. "With over-the-counter sale of Plan B, at least they could do this. And young women are more likely to be raped. We want these women to get medical care, but most do not do it quickly."

Plan B Fact and Fiction

Plan B should be taken within 72 hours (three days) of unprotected intercourse. When taken within 72 hours, it decreases the chance of getting pregnant by 89% -- from 8% without Plan B to 1% with Plan B.

Plan B is even more effective when taken within 24 hours. Effectiveness decreases the longer a woman waits to take it.

Plan B is not an abortion pill. It contains levonorgestrel, a synthetic version of the hormone progestin. The abortion pill RU-486 contains a completely different drug.

Plan B works mainly by preventing release of eggs from a woman's ovary, although it may also prevent sperm from fertilizing the egg. But if a fertilized egg already has been implanted, the pregnancy continues normally even if a woman takes Plan B.

Plan B may cause side effects. The most common side effect is nausea, which occurs in about a quarter of women after taking Plan B. Other side effects may include abdominal pain, fatigue, headache, and heavy menstrual bleeding.

An older version of Plan B required a woman to take two pills 12 hours apart. The current Plan B One-Step formulation is a single pill taken only once, as soon as possible after unprotected intercourse but no longer than 72 hours later.

Plan B isn't the only emergency contraceptive. Ella, from HRA Pharma, is a different medication that prevents pregnancy when taken up to five days after intercourse. Ella is available only by prescription; HRA has not sought over-the-counter approval from FDA.

And contrary to the fears of some, women and teens with access to Plan B or Ella do not use the drugs as a regular contraceptive, Yale’s Stanwood says. And teens who can get these drugs do not have sex more than teens who can't get them, she says.

"Luckily we don't need to have these fears," Stanwood says. "There is this urban legend women will abandon regular birth control, and we have studies to show that is not the case. Women with Plan B do not abandon plan A or take more risks. It's like giving them a fire extinguisher -- it doesn't make them start more fires."